Your starting point in the fascinating world of propellers. Standard scale props are intended to suit the widest range of scale boats from tramp steamers to fast patrol boats. They are designed mainly with electric motors in mind, but will run quite nicely on steam. They all have fine to medium pitches so that, when matched to the recommended motor, they will deliver excellent handling, thrust and battery duration. This is possible because the blades are not soldered on at a 'guesstimated' angle but carved to the correct geometry and then precision cast in one piece. Nasty characteristics such as torque roll and non-existent reverse thrust are eliminated. Where the precise style of a prop is not known, these will still set off the model handsomely, and will arouse few complaints even from purists. Available in three, four or five blades.
Reference numbers for this range give the diameter in inches followed by the pitch in inches. Eg. 2022 is 2" dia. x 2.2" pitch. There are a few anomalies: 0550 is 0.5" x 0.5", 0675 is 0.6" x 0.75", 0751 is 0.75" x 1", 0871 is 0.87" x 1", 27522 is 2.75" x 2.2" and 8120 is 8.0" x 12.0"
The typical thread types for these props are M2 (for very small props), M3, M4 and M5. You can specify another thread type if you know exactly what you need.